This invention relates to the chemical treatment of domestic, commercial or industrial waste water to remove various solids and contaminants therefrom. More particularly, the invention involves a relatively high degree of contaminant removal from municipal sewage and other types of waste water having a high total solids content (at least 200 mg/l) so that the resultant effluent can be used immediately for commercial purposes or even rendered potable if desired.
Many chemical processes for the purification of impure or turbid water of the type ordinarily found in rivers and streams have been proposed in the past. Examples of such processes may be found in Keith, Jr. et al, U.S. Pat. No. 3,557,341, Sebald, U.S. Pat. No. 2,355,564, Farnham, U.S. Pat. No. 2,964,466, Kratz, U.S. Pat. No. 3,017,347 and Massatsch, U.S. Pat. No. 1,107,199. However such impure or turbid water is not comparable to "waste water" as the term is used herein, the latter having a total solids content normally exceeding that of the former by a factor of roughly 10, and having bacterial populations which are enormous compared to the bacterial population of mere impure or turbid water. Because of the very high solids and bacterial content of such waste water, techniques designed for mere water purification such as those shown in the foregoing patents are inadequate for treatment of waste water. This is primarily due to the fact that in water purification processes the precipitating agents are normally added at once, as shown for example in the Keith, Jr. et al or Sebald patents, rather than at different stations with solids removal steps interposed between successive additions of the precipitating agents. It has been found that the much larger mass, and greater ranges of sizes and complexity of waste water solids, when treated in such a manner, have a severe diluting effect on each of the precipitating agents by forcing their distribution among a large mass of solids for which they are not effective treatment agents, as well as among those solids for which they are effective. The effectiveness of each precipitating agent is thereby reduced, requiring a greater amount and higher cost of each agent to compensate for its reduced effectiveness. In addition the time required for sufficient solids removal is thereby unnecessarily prolonged. These disadvantages apply particularly in a system where settling, which is much less expensive than filtration, is utilized to remove the major solids.
Chemical systems have also been devised for the treatment of sewage or other types of waste water having relatively high total solids and bacterial content, as shown for example in Travers, U.S. Pat. No. 1,672,584, Brent et al, U.S. Pat. No. 3,687,646, Rice, U.S. Pat. No. 3,171,804, and Luck, U.S. Pat. No. 3,801,499. These however also suffer from the problem previously described with respect to the foregoing water purification systems, in that although multiple precipitating agents are added to the waste water they are also added substantially at once without the intervention of solids removal steps between successive additions. Thus the dilution of effectiveness and lack of economy with respect to the precipitating agents remains.
In addition, none of the prior processes suggests any means for insuring that the pH of the final treated effluent will automatically be in the desired neutral range without the costly addition of further chemicals whose only purpose is pH adjustment. This problem arises particularly in the chemical treatment of waste waters if the formation of nitrates is to be avoided during the treatment process, such nitrates being unduly expensive to remove once formed. To avoid the formation of such nitrates, it has been determined that the pH of the waste water must be raised to a relatively high level immediately at the beginning of the treatment process to kill nitrifying bacteria at the outset. The Luck, U.S. Pat. No. 3,801,499 does raise the pH of the waste water to over 11 to destroy pathogenic bacteria at the beginning of the treatment process, but does not show how to obtain a neutral final effluent (i.e. in the pH range of 6-8.5) without the addition of pH adjustment chemicals which have no other purpose in the process than to adjust the pH, thereby creating additional expense.
Finally, although the Sebald water purification process teaches the mixing of solids, precipitated from impure water, with further raw untreated water to provide more "complete mixing" prior to the entrance of the raw water into the treatment chamber, neither Sebald nor any other patents suggests the possibility of utilizing previously precipitated solids as replacements for treatment chemicals, particularly coagulant aids, whereby at the start of a process a normal amount of treatment chemical is used and then, after the process has progressed to the point where sufficient precipitated solids are produced, the precipitated solids are used to replace a portion of the original chemical in the treatment of further raw water, with the amount of the original treatment chemical required to achieve the same results being thereby reduced.
The foregoing deficiencies of the prior art have generally required such a high cost of chemicals for treatment of a given volume of waste water that such operations have not been commercially feasible. Accordingly there exists a great need for a chemical waste water treatment system which overcomes the lack of economy inherent in the previous systems while producing equal or better results.